The atmosphere is currently composed of 21% oxygen and 0.03% carbon dioxide. Photosynthesis, cellular respiration, and other processes such as fossil fuel combustion combine to create the composition of the atmosphere.
One way is via deforestation because deforestation essentially destroys nature's potential of making O2 via the process of photosynthesis. Deforestation is the cutting down of trees to improve the value of tracts of land by making space for farms usually. Trees are a large source of oxygen and also remove carbon dioxide from the atmosphere. by cutting down trees, you limit the amount of carbon dioxide being removed from the air and, usually, the trees are burnt after being cut down in a 'chop and burn' method, this releases the carbon in the tree which reacts with oxygen because of the heat and moving into the atmosphere. As of such, not only does deforestation remove sources of oxygen and carbnon dioxide sinks (a 'resivior' of CO2) it also adds excess oxygen into the atmosphere.
Industrialization increases the level of carbon in the atmosphere, which is a greenhouse gas. An increase in greenhouse gases causes global warming.
Since most of the oxygen in the atmosphere comes from photosynthesis aquatic species such as algae; polluting the ocean, lake, or other body of water could kill these species and, in turn, decrease the amount of atmosphere oxygen.
I hope this will give you a head start in answering your question! Good Luck
.